We investigated whether microalbuminuria was associated with the metabolic syndrome by comparing the strength of the association between microalbuminuria and the syndrome as a whole and its individual components. This investigation included 5659 women and men aged 20 to 80 years from the cross-sectional, nationally representative, Third National Health and Nutrition Examination Survey (NHANES III: 1988-1994). Metabolic syndrome was defined as any three of the following: increased waist circumference, increased triglycerides, decreased HDL cholesterol, increased blood pressure, or high fasting glucose. Microalbuminuria was defined as urinary albumin/creatinine ratio of 30 to 300 mg/g. Microalbuminuria was present in 7.8% of women and 5.0% of men. Log linear analysis revealed a significant association between the metabolic syndrome and microalbuminuria in both genders (women chi(2) = 44.1; men chi(2) = 59.6; P <.0001 for both). Microalbuminuria was more common in both women (odds ratio [OR] = 2.2; 95% confidence interval [CI] 1.44, 3.34) and men (OR = 4.1; 95% CI 2.45, 6.74) with metabolic syndrome compared to those without it; 34% of women and 42% of men with microalbuminuria also had metabolic syndrome. After adjusting for other components of the metabolic syndrome, hypertension demonstrated the strongest association with microalbuminuria in both women (OR = 3.34; 95% CI 2.45, 4.55) and men (OR = 2.51; 95% CI 1.63, 3.86). Microalbuminuria and metabolic syndrome are associated in a large, nationally representative cohort, possibly due to early renal effects of hypertension, and it may be useful to consider microalbuminuria as a component of the metabolic syndrome.